Phased array antennas comprise clusters of dipole energy radiators, for instance. Typically, these dipole radiators are arranged in a planar configuration. Each dipole radiator is driven by variable phase-shifting circuitry such that the array of dipole radiators sweeps a composite beam of radiated energy across a field of view. For example, if dipole radiators in an array are driven with a linear progression of phase shifts, the array of these radiators produces a phase front which travels at an angle to the array.
Phased array antenna systems are currently used in radar and communication systems and, for many applications, are preferred over conventional reflector antenna systems. Phased array antenna systems are capable of electronic scanning, are conformable to the surface of a vehicle carrying the system, such as an aircraft, and are compact. Phased array antenna systems are being considered for use aboard aircraft such as those used for the Airborne Warning And Control System (AWACS). A phased array antenna system would eliminate the current need for the large rotodome which sits atop an AWACS aircraft, and would thus eliminate the drag on the aircraft that the rotodome produces.
Radar targets are more readily visible in frequency bands where dimensions of targets are resonant. For many targets, this band includes the combined VHF and UHF band. (VHF is considered to extend approximately between 30 MHz and 300 MHz. UHF is considered to extend approximately between 300 MHz and 900 MHz.) For instance, a radar target such as a cruise missile having primary dimensions on the order of a small number of wavelengths in the VHF/UHF band has multiple resonances there and so reflects strong radar signals in that band. Maximum detection considerations tend to favor the low frequency end of this band. However, signal interference considerations tend to favor the UHF band which therefore becomes the band most favored. A horizontally polarized VHF/UHF band radar system more easily detects a radar target, such as a cruise missile or small aircraft, because such targets generally are oriented horizontally. However, a radar system must have a very large VHF/UHF band antenna to sufficiently track a radar target and provide high target resolution. Such a very large radar system aboard aircraft such as an AWACS aircraft would be impractical.